Line concentrator system



July 30, 1963 J. A. HARR l-:TAL

LINE coNcENTRAToR SYSTEM 4 Sheets-Sheet 1 Filed Dec. 24, 1959 lmfilLsmslflfI /N VEA/raps ATTORNEY July 30, 1963 J. A. HARR ETAL LINECONCENTRATOR SYSTEM 4 Sheets-Sheet 2 Filed Dec. 24, 1959 IIIIIIvTII w Wwww Hm. Aw. M n J./. A m

W NSK J. A. HARR ETAL LINE CONCENTRATOR SYSTEM July 30, 1963 4sheets-sheet s Filed Dec. 24, 1959 /VENTORS J- A. HA/PR By r N. OWY

ATTORNEY.

July 30, 1963 J. A. HARR ETAI.

LINE coNcENTRAToR SYSTEM 4 Sheets-Sheet 4 Filed Deo. 24, 1959 ATTORNEYUnited States Patent Office 3,099,717 Patented July 30, 1963 3,099,717LINE CONCENTRATOR SYSTEM John A. Harr, Morris Plains, and Terrell N.Lowry,

Boonton, NJ., assignors to Bell Telephone Laboratories, Incorporated,New York, N.Y., a corporation of New York Filed Dec. 24, 1959, Ser. No.861,809 17 Claims. (Cl. 179-18) This invention relates to telephonesystems and more particularly to arrangements for servicing customersconnected to the same telephone line concentrator in a telephone lineconcentrator system.

The concentration of telephone lines and lthe connection of 'arelatively larger number of substation lines to a relatively smallernumber of telephone switching paths [has characterized telephoneswitching since its earliest development,

In the past, automatic telephone switching equipment in a centralolii-ce was arranged to concentrate the traftic to the succeedingequipment in the office. In this manner la portion of the equipment wasnot provided on a per substation line basis and instead, for economicconsiderations, was designed merely to process the expected peak load atthe required grade of service.

Recently, however, the telephone switching art bas been infused with anincreasing interest in remote line concentration, i.e., an arrangementin which substation lines are concentrated at a locale remote from thecentral oflice. In remote concentrator arrangements a lesser number ofchannels or trunks than the number of substation lines is utilized fromthe point of concentration to the central office. The economic yandother benefits that acc-rue from reduction of cable costs and otheroutside plant expenditures in remote concentrator arrangements are ofsubstantial signilicance and have spurred developmental eiforts in thisarea.

As an illustration of a recent embodiment of a twostage telephone lineconcentrator system of advanced type, reference may be made to anapplication lof Harr- Lowry-Nehama-Ridinger Serial No. 848,595, tiledOcto ber 26, 1959.

The advantages which -derive from the utilization of a system of thisnature are manifold and it is inevitable that the impact in thetelephone switching field should be experienced in many quarters.

`For example, the area of telephone private branch exchanges, which inrecent years has undergone extensive growth, is uniquely adapted toenjoy the advantages which inhere in concentrator systems and relatedelectronic telephone switching systems.

IIn conventional telephone private branch exchange arrangements, it iscustomary Ito provide on the premises of the telephone private branchexchange subscriber, switching facilities for terminating all of theextensions or telephone circuits included within the telephone privatebranch exchange or PBX. These termination and switching facilities maybe either 'automatic or manual in nature. In either event their ultimatefunction is to connect a particu-lar extension circuit to the centraloffice over a number of trunks which are customarily less in number thanthe number of extension circuits.

Although these arrangements are completely operative and useful asevidenced by their increasingly widespread acceptance, `a number ofmanifest disadvantages inhere in the use of conventional PBX systems. For example, since each telephone circuit is terminated at the PBX andhas no unique appearance at the central oliice, direct distance dialingfrom a PBX circuit represents a major difficulty. The problem isgrounded in .the nature of the PBX circuit termination and theavailability of a connection to the office on -a shared basis only.Under these circumstances an effort to obtain the identiiication of theparticular circuit origin'ating a [direct distance call on .an automaticbasis is a formidable obstacle which in the past has been a substantialdeterrent to the implementation of direct distance dialing in PBXsystems.

Another signilicant difficulty which is perhaps more serious from .theconsideration of traffic liow is that an incoming call to a PBX cannotordinarily be completed on :an automatic basis to the called extensionwithout the intervention of an operator at the caille-d PBX.

Since the percentage of .traiiic origin-ating and terminating Vat PBXextensions is becoming proportionally more significant, the manualintervention by the operator is manifestly restrictive and has athrottling effect on the ilow of otherwise automatic traic. Themagnitude of this problem may be gauged by examining the recent-attempts to provide automatic in-dialing to PBX extensions without thenecessity of operator assistance. Although completly operative anddesirable, these arrangements are often predicated on the incorporationof sophisticated and costly switching arrangements.

It has therefore seemed desirable to utilize the concepts of remoteconcentrator switching and the supe-rior switching capabilities ofelectronic telephone switching systems to radically change thephilosophy of telephone PBX switching. Thus in lieu of the prosaicarrangement whereby each telephone extension in a PBX is terminated atthe local on-premises switchboard or the local PBX automatic switchingsystem each PBX extension is connected to lthe central office in amanner relatively indistinguishable from that of a private linesubscriber, Le., non-PBX subscriber.

Tlhus, through a single abrupt departure from conventional switchingtechniques all of the usual disadvantages which inhere in PBX systems asenumerated above are overcome since each PBX substation line ap pears inthe same form as a private line and the same bil-ling and identificationfacilities are available for the PBX extension @as for the privatelines. In a similar manner the obstacles attendant on in-dialing to .aPBX extension are eradicated since the same automatic dialing ,techniquemay be employed for PBX extensions as for private lines.

It will be :appreciated immediately, however, that in the course ofconnection of all PBX extensions to the central office through a remoteconcentrator system, certain manifest inefficiencies occur. A strikingcase is presented in a two-stage concentrator, i.e., where thesubstation lines are concentrated in a primary stage of concentrationand extended over links to a secondary stage of concentration from whicha relatively fewer number of trunks extend to the central office. In asituation where two PBX extensions are extended to the same remoteprimary concentrator, an intra-PBX call, since it must be completedthrough the central oice, will necessitate the use in the originatingconnection of a line to the primary concentrator, a link to thesecondary concentrator, a trunk to the central office, an intra-officetrunk or channel, and in the terminating connection a separate trunkback to the same secondary concentrator, a separate link back fto thesame primary concentrator and finally, the called line. This conditionis necessitated in each instance when a calling PBX customer makes anintra-PBX call to a customer on the same remote primary concentrator.The necessity of extending paths from the calling customer to thecentral oflice and rfrom the central office to the called customer taxesthe existing facilities between the primary concentrator and the centraloflice and limits the usage of rthese channels by other wouldbe callers.

Itis therefore an object of this invention to provide for telephoneconnections *between customers having a high 3 community of interestwithout the `continued utilization of trunks to and from the centraloce.

A further object of this invention is to permit telephone communicationbetween selected subscribers connected to a particular primaryconcentrator over circuits local to said concentrator.

IStill another object of this invention is an arrangement for providingfor telephone connections between subscribers connected to differentconcentrators without the continued utilization of channels to thecentral oflice.

A further object of this invention is to supervise and controlintra-concentrator and interconcentrator connections.

These and other objects and features of the invention may be realized inone illustrative embodiment in which 'special intra-primary skew, localor reentrant links are provided in the concentrator network. The skewlinks provide connections between two link terminations at the primaryconcentrator in a two-stage concentrator switching system. 'I'heseadditional skew links are located wholly within the concentrator.

When a PBX customer connected to a particular concentratordials outsidethe PBX, the call is handled in the conventional manner described inHarr et al. referred to above. If the PBX customer makes lan intra-PBXcall, however, the central ofHce participates conventionally untilanswer takes place. At this time both links and both trunks are releasedand a local connection employing a ske-w link is established within theprimary network,

Incoming calls to the PBX may be dialed directly since each PBXextension is terminated in the same manner as private substation lines.

Additional embodiments of the invention include single stage versions ofthe intra-primary skew, local or reentrant links and moreover includeinterprimary skew, local or reentrant links for joining subscribershaving la high community of interest but connected to different primaryconcentrators which in turn are connected to a single secondaryconcentrator. Still another embodiment includes a single stagearrangement of concentration having interprimary skew or reentrantlinks.

A feature of this invention includes a primary concentrator in atwo-stage concentrator system having a group of skew or reentrant linkswithin the concentrator.

Another feature of this invention includes skew or reentrant linksjoining diiferent primary concentrators connected to the same secondaryconcentrator.

Still another feature of this` invention is a single stage concentratorincluding a group of skew or reentrant links within the concentrator.

A further feature of this invention includes a single stage concentratorarrangement in which a plurality of concentrators are joined by skew orreentrant links to provide connections between subscribers having a highcommunity of interest.

'Ihese and other objects and features of the invention may be morereadily comprehended from an examination of the following specificationand attached drawing in which:

IFIG. 1 shows a portion of a two-stage line concentrator systemincluding a skew or reentrant link;

FIG. 2 shows a two-stage concentrator including interprimary andintraprimary links;

FIG. 3 shows a single stage concentrator including intra-primary links;and

FIG. 4 shows a single stage concentrator including interprimary andintra-primary links.

For purposes of clarity of presentation, only those portions of theconcentrator system described Lby Harr et al. supra, which are essentialto a comprehension of the present invention have been disclosed indetail. For an explanation in depth of the operation of the scanner andrelated equipment, reference may also be made to an application ofCirone-Harr-Lowry-Ridinger, Serial No. 824,-

. 4 294, filed July 1, 1959, all of which applications are herewithincorporated by reference.

General Description of Major Components Referring now to FIG. l it maybe observed that a number of substations 11 and 12 are illustrative orsymbolic of as many las 58 substations connected to a particular primaryconcentrator over individual lines. rPhe primary concentrator stageshown in abbreviated format includes a full access switch for connectinga maximum of 58 subscribers to ten links which join the remote primaryconcentrator to the remote secondary concentrator. In FIG. 1 the linksare symbolically represented by links 19 and 23'. It is understood,however, that in practice a greater number of links would beincorporated in the manner shown for links 19 and 23'.

The secondary concentrator stage includes a partial access switch forservicing up Ito 200 links which may be selectively connected to amaximum of trunks that connect the remote secondary concentrator to thecentral oice. These trunks are represented in FIG. l by trunks 20 and 24which extend from the secondary concentrator to the distribution networkin the central ofce.

Both `the primary .and secondary networks may illusnatively be one-wireswitches employing PNPN transistor crosspoints. Conversion from two-wiretransmission to :one-wire transmission is accomplished by means oftransformer coup i' g. 'Iialking battery is :supplied in the primaryconcentrator and crosspoint holding power is also applied locally.

A connection through the remote network is established by selecting anidle path through both stages of concentration. Identifying numbers forthe selected and trunk `are then transmitted to the primary andsecondary concentrators with a mar order.

The remote networks are released from the link side cf the remoteprimary concentrator `and the trunk side of the secondary concentrator.Complete records of each connection are stored in a temporary routememory at the central office and the identification lof the link andtrunk to be disconnected is thus readily available.

The lines land links are scanned on a periodic basis at the remoteprirnary concentrator to detect service requests, :auswers anddisconnects. When any one of these conditions obtains an appropriatecontrol signal is delivered lto the central cfice on the next succeedinginterrogation or advance pulse.

The over-all arrangement of the concentrator shown in FIG. 1 is based ona `synchronous control system. Each of the lines and links is examinedsequentially under control of 'a counter 72 at the remote primaryconcentrator. A master counter (not shown) in the remote concentratorcont-rol at the central oiice is stepped in synchuonism with thecounters in the primaries.

The advantages which inhere in the use of a synchronous control systeminclude simplied idenriiication of a customer initiating a servicerequest. lIn the present arrangement it is essential only for the remoteprimary concentrator to forward control information to the centual oiceindicating :a service request. At the central office control equipmentin the remote concentrator con- .trol examine fthe line number counterto identify the number of the line at fthe remote primary concentratorrequesting service.

Switching functions are performed in response to information -gleaned byline scanner 71 and link scanner 25. Line scanner 71 may illustrativelybe a two-stage transistor Itree or take any other suitable form which iscapable of providing la temporary connection between 60 illustrativeinputs to ,the scanner (including one for |each line to be examined sandtwo contnol inputs) and la transmission path to the central loice. Thelink scanner 25 may take a related form which is adapted to providesequential connections from each of the illustrative ten links 19-23 tola transmission channel to the central oice.

establish existing skew link connections under control of the centralofl'ice.

Secondary Concentrator Equipment Heretofore, the :control equipment inthe secondary concentrator has not been referred to but it will suiiicefor the purposes of the present explanation to appreciate that the linkmark selector 16 is similar in structure to link mark selectors 14 land23 and may take the form of a transistor tree or any other suitablerorrn for placing marking potentials on individual links in response tosignals from the central oice. Similarly, trunk mark selector 17 mayalso be a transistor tree or other apparatus 'for establishing temporaryconnections to each of the trunks to permit the application of markinglpotentials individual thereto in response to signals from the centraloffice. The trunk release selector 27 is similar in form and function totrunk mark selector 17 with the distinction residing in the purpose ofapplying releasing potentials to selected associated crosspoints therebydeenergizing the crosspoints and discontinuing previous connections. Aparticular crosspoint in the secondary concentrator, for examplecrosspoint 18, is energized by transmission of appropriate coded signalinformation to link mark selector `16 in the secondary concentrator andtrunk mark selector 17 to apply marking potential to both sides ofcrosspoint 18 thereby energizing the crosspoint. v'For details of thetransmission of -control signals from the central oice to the primaryand secondary concentrators to actuate the control equipment thereat,reference may be made to Harr et al. referred to above.

Filter 73 connected in series between the two matrix appearances of skewlink 32 functions to prevent the leakage of supervisory signals betweenlinks and is adapted to present no substantial impedance to the llow ofvoice frequency signals. It is important to observe that skew link 32 isembodied wholly within the primary concentrator and has no portionswhich connect to the secondary concentrator in contradistinction tooffice links 19 `and 23 whose ultimate function is to establish pathsfrom the primary concentrator to the secondary concentrator.

Having thus described the major equipment, a description of thefunctional operation of the invention in detail will follow.

T ivo-Stage Concentratr-lntraPrimWy Skew Links For purposes ofillustration a typical call in-itiated by Ia subscriber at substation 11to a subscriber connected |to the same remote primary concentrator atsubstation 12 will be explained. It will be assumed that the twosubstations 11 and 12 are connected -to fthe same private branchexchange or PBX and that the call is in eiect an intra-concentratorcall. When subscriber 11 goes otiliook the line scanner 71 will observethe service request Iand transmit an indication thereof to the centraloice. Common control equipment in the central oice recognizes theservice request and identifies the primary concentrator and secondaryconcentrator to which the line is connected and also 4the line number.

After establishing the service request identication, common controlproceed-s to establish Ia connection between the central oliice andthecalling substation in order that the subscriber may key or `dial thedigits of the called number. Common control by reference to a memoryselects an appropriate route to eilect a connection between substation11 land the central oidce including an idle link and an idle trunk. Theremote concentrator control equipment at the `central olli'ce is thenenergized to transmit the necessary ycontrol impulses to effect 'theconnection. These impulses are delivered to the primary andfsecondaryconcentrators Iand energizes the line mark selector 13 and the link markselector 14 to operate a selected crosspoint in the primaryconcentrator, as described above. Similarly, the link mark selector 16and trunk mark selector 17 are also actuated by the control impulsesfrom the central ofce remote concentrator control to operate a selectedcrosspoint 18 in the secondary concentrator.

The subscriber at substation 11 is now provided with a path through theremote primary and secondary concentrators to the central officeincluding crosspoint 15, link 19, -crosspoint 18 and trunk 20. Equipmentlocated in the central office is 'actuated to effect Ia connectionbetween trunk 20 and a digit detector (not shown) to receive signalsfrom substation 11 indicative of the called number, in this casesubstation 12.

A connection is thereby established between the subscriber at substation11 -and the digit detector, which latter transmits dial tone :to thecalling subscriber. The subscriber now keys or dials the vcalleddirectory number into the digit detector.

The necessary translation from the received directory number digits toestablish the terminating stat-ion equipment number is made and in theassumed illustration common `control "arrives at a translationrepresenting the equipment number of called substation 12. As a resultlof the translation infomation pertaining to the identification of thesecondary concentrator, primary concentrator and line number of thecalled station are obtained. Preparations are thereupon made toestablish a terminating call to the latter station.

The central ofce transmits :signals to the primary concentrator toladvance the line mark selector 13 to the lcalled line 12. If substation12 is idle an appropriate idle link 'and trunk for the terminatingconnection to substation 12 are selected. Further control signals arenow delivered by the central oi`n`ce to the line mark selector 13 andlink mark selector 14 to actuate crosspoint 21. Additional controlsignals are transmitted to the secondary concentrator to operate markselector 16 and trunk mark selector 17 to energize crosspoint 22 therebycompleting a path from the `called substation 12 to the central ofliceincluding `crosspoint 21, link 23, crosspoint 22 and trunk 24. If thecalled `line is still onhook, common control in the central oflicearranges a ringing connection to the lcalled substation 12 and anaudible ringing tone is `applied to the trunk connected to substation11.

When the called substation answers and the switchhook contacts thereatare closed, this condition is observed at the link scanner 25 and asignal is transmitted to the central oilice indicating ian answercondition. yIn consequence, the central olce discontinues ringing.

At this time common control will refer to memory or storage devices todetermine whether `a shorter or local path can be established betweenthe calling and called substations. In the assumed illustration theequipment number identification of substations 11 and 12 will indicatethat they are connected to the same PBX and the same primaryconcentrator. In View of this observation, the remote concentratorcontrol prepares to discontinue the established connection from theoriginating party to the central office yand from the central otiice tothe called party and to substitute therefor ya local or skew link.

The procedure for releasing the crosspoints in the primary and secondaryvconcentrators is similar to that described in the preceding section andin further detail in Harr et al. supra. A series of code pulses istransmitted -to link release selector 26 Iand to trunk release selector27 which deenergize crosspoints 15 and 18, respectively, thereby openingthe originating connection.

It now remains to establish a connection over a skew link `in theprimary concentrator between the calling party at substation 11 and thecal-led party `at substation 12. In this instance code signals laretransmitted to the skew link mark selector 2S and to the line markselector 13 to energize crosspoint 29 thereby `completing a path betweenthe originating party at substation 11 and one end of skew link 32. Atthis time one-half of the original '37 are symbolically represented by4links 40, etc.

call has -been supplanted by a connection to skew link 32.

Having transferred one-half of the call, namely the originatin-gconnection, it is now necessary to process the remaining half of thecall. The same procedure is now followed with respect to the terminatingsubscriber and the :previous connections through the primary andsecond-ary concentrators at lcrosspoints 21 land 22 are discontinued.Appropriate coded signals 4are transmitted to link release selector 26and trunk release selector 27 to deenergize crosspoints 21 and 22thereby disestablishing the terminating connection. Common control nowprepares additional `orders lfor the remote concentrators which aredelivered :to skew link mark selector 28 and line mark selector 13 toenergize crosspoint 30 thereby connecting the terminating party to theother half of skew link 32. When the originating and called parties areconnected to opposite -sides `of skew link 32 the two subscribers mayconverse indefinitely and no further action is required Ias long asconversation ensues. The skew links are scanned by skew link :scanner 33in the same manner that the conventional office links are scanned bylink scanner 25.

When either subscriber connected to the skew link 32 hangs up thecondition is observed by skew link scanner 33 and a signal indicativethereof is transmitted to the central office remote concentrator controlas described in Harr et al. supra. A release order is formulated bycommon control and appropriate code signals are transmitted to actuateskew link release selector 34. Assuming that the calli-ng partydisconnects first, skew link release selector 34 is actuated todeenergize crosspoint 29. The remaining party -at substation 12 willcontinue connected to skew 32 until he too goes on-hook. When skew linkscanner 33 detects the on-hook condition at substation 12 an appropriatecontrol signal is transmitted to the central office and common controlformulates a release order which is transmited to link release selector34 to deenergize crosspoint 30. When both parties have disconnected thecircuit is returned to normal.

T wo-Szage Conccntrafor Including Interprmary Skew Links In reference toFIG. 2 it may be observed Ithat a number of substations are connected4to each of the primary concentrators 35 and 41. These substations areillustrative of a considerably larger number which may be connected tothe concentrators. Here again the primary concentrator is shown inabbreviated form and includes a full access switch for connecting 58substations, [for example, to ten links which join the remote primaryconcentrator 35 to the secondary concentrator 37. These ten links areillustratively represented by link 36, etc. The ten links joiningprimary concentrator 41 to secondary concentrator The secondaryconcentrator stage shown in outline form is similar Ito that shown :inFIG. 1. Moreover, trunks 38 and 39 are illustrative of up to 101'0trunks that connect the secondary concentrator to the central otiice.

In referring to FIG. 2, primary concentrator 35 is assumed to be similarinsofar as equipment is concerned as the prirnary concentrator shown indetail in FIG. 1. For clarity of presentation the line number counter72, line scanner 71 land line mark selector 13 Ihave been omitted. Inaddition the control equipment associated with the oice link matrix asshown in FIG. 1 including the link scanner 25, link mark selector 14 andlink release selector 26 have also been omitted.

An examination of FIG. 2 indicates that the signiiicant and compellingdeparture from FIG. 1 is in the utilization of interprirn'ary skew links78. These links are utilized to couple the interprimary matrices ofprimary concentrators 35 and 41.

I The interprimary skew links 78 are in all respects similar to theintra-primary skew link 32 shown in detail in FIG. l with thedistinction residing in the premise 10 that the interprimary ske-w linksphysically depart from the primary concentrator conlines and areutilized to join geographically distinct primary concentrators.

Thus in a comparison of the intra-primary skew link arrangement of FIG.1 and the additional equipment shown in FIG. 2, it is apparent that thelatter embodiment includes :all of the equipment of FIG. 1 including theskew link control equipment. The embodiment of FIG. 2 includes,moreover, a separate interprimary matrix in each primary concentratorland interprima-ry skew links 78 for coupling the interprimary matrices.

In order to illustrate the operation of the embodiment of FIG. 2 asimilar illustrative call will be traced. If it is assumed that acalling subscriber at substation 11 seeks to effect a connection to acalled subscriber at substation 12 in FIG. 2, a connection may beextended from the calling subscriber at sulbstati-on 11 over crosspoint76, link 36, secondary concentrator 37 and trunk 38 to the centraloffice, as explained above with regard to FIG. 1. This circuit pathcomprises the originating connection, A similar terminating connectionmay be made to the called lsubscriber at substation 12 over trunk 39,secondary concentrator 37, link 40 and crosspoint 77. When the centraloffice has completed the originating and terminating connections andringing is applied to the called party substation it will be observedalso as explained in FIG. l that the connection may be satisfied by ashorter or local path which in this case is not `an intra-primary skewlink as in the previous example but is instead an inter-l primary skewlink.

The originating and terminating connections are released ;as describedabove by transmitting control signals to the link release selectors andthe ltrunk release selectors. A local connection is then effectedbetween substation y11 and substation lf2 over interprimary skew link 78including crosspoints 79 and 80. The manner of establishing the skewlink path is similar to that explained above with respect to FIG. 1 andincludes the transmission of appropriate control signals to the linemark selector, not shown, and the skew link rnark selector 28 ofconcentrators 35 and 41. It may be observed that the interprimary skewlink path 78 is in all respects similar to intra-primary skew link path32 lof FIG. l with the exception that in the former instance one-half ofthe link termination is in primary concentrator 35 and the other half isin primary concentrator 41. It Iwill be noted that in FIG. 2 thetransformer coupling terminations have been omitted to avoid complexitybut it lis assumed that they are in all respects similar to that shownin detail in FIG. l.

After a local interprimary skew link connection has been effected, lapath exists between the calling subscriber at substation 11 and thecalled subscriber at substation 12 which does not include any higherlevel channels such as links 36, `40, etc. to the secondary concentratoror trunks 38, 39, etc. to the central cnice. These latter channels foradditional traffic are thus freed for addi-tional tratiic.

Conversation may now ensue indefinitely between the calling and calledparties at subis/tations 11 and 12, respectively, over the inter-primaryskew link path 78. When the parties disconnect, release of thecrcsspoints 79 and 80 is performed in the manner described above withreference to FIG. 1. In this instance skew link scanner 33 observes thehangup condition and a signal indicative thereof is transmitted to thecentral o'ice remote concentrator control as described in Harr et al.supra. A release order is again formulated in common control andappropriate code signals are transmitted to actuate skew link releaseselector 34 in the appropriate primary concentrator 35 or 41. As in theformer illustration the second party to hang up will remain connectedtoskew link 78 until the second substation goes onehook. A similarprocedure is then followed to disconnect the remaining panty. When bothparties have been disconnected the circuit is returned to normal.

lllVA Single Stage Concentrator Including Intra-Primary Skew LinksReferring now to FIG. 3 a circuit is shown which is in all respectssimilar yto that of FIG. 1 with the modication residing in the absenceof multiple concentration stages. Under these conditions, concentrator81 is joined by links 43, 44, etc. to the central oce and skew link `45is utilized in the manner described in FIG. 1.

Here again links 43 and *44 are illustrative of a considerably largernumber of links that connect concentrato@- 81 to the central oice.Moreover, substations 111 and 12 are symbolic of a plurality ofsubstations that may be connected to the concentrator network. Theconcentrator netlwork itself is shown in abbreviated format `and it isunderstood that the office matrix includes sufficient cross-points toconstitute a full access switch lfor connecting all of the substationlines to all of the links to the central otlice. The intra-primarymatrix shown in concentrator 81 is designed to afford access from eachof the lines to each of the skew links, only one of which, skew link 45,is shown in detail. Moreover, as in the case of FIG. 2, the controlequipment or the office matrix has been omitted to reduce complexity.

To explain the operation and functioning of the embodiment of PIG. 3 itwill be assumed that a -customer at substation '1"1 is seeking to effecta connection to a subscriber at substation f12. Connections will beextended in the usual manner between the central office and substation11 over crosspoint 83 and link 431y `for the originating connection andover crosspoiut 84 and link 44 -for the terminating connection. Ringingsignal is thenapplied to the called substation and when it is observedat the central office that the connection may be satisfied by a localpath the originating and terminating connections are disestablishedthrough appropriate control orders transmitted to the skew link releaseselector and an idle skew link path, for example skew link 45, isemployed in the manner described for FIG. t1 to connect substations 11and 12 over crosspoints 85 and 86.

Supervision over the intra-primary skew link 45 is maintained `by theskew link scanner.

As before when either subscriber connected to the skew link 45 hangs up,the condition is observed by the skew link scanner and a signalindicative thereof is transmitted to the central office. A release orderis then formulated and .appropriate code signals lare returned toactuate the skew link release selector to deenengize the' appropriatecrosspoint. When both panties have disconnected the circuit is restoredto normal.

Single Stage Concentrator Including Inter-Primary Skew Links In FIG. 4 asingle stage version of the embodiment of IFIG. 2 is shown which issimilar in all respects to that of FIG. 2. with the departure residingin the utilization of single in lieu of double concentration. It will beassumed cior the purposes of the description of FIG. 4 that the prirnaryconcentrators l35 and 41 are similar in al1 respects to that of FIG. 2.Thus concentrator 35 includes an oftice link matrix, an intra-primarymatrix and an interprimary matrix. Here again the only control equipmentshown is that `for the skew link matrices. As usual the few substationsconnected to the concentrators are representative of a larger number asare'the links 36 and 40 connected to the central office. Thus the linksbetween the primary concentrator S5 and the central oflice may number asmany las ten. In addition, the interprimary skew links 78 and 87 arerepresentative of a larger number.

In tracing a typical call ybetween a customer yat substation l1=1 and acustomer at substation 12 the conventional originating and terminatingconnections over links 36 and 40` are utilized to join the originatingand terminating substations to the central office. When ringing signalis applied and it is observed that a local path may be implemented I(inthis case interprimary skeW link 7S), the :appropriate release ordersare transmitted to the concentrators 3S and 411 to effect release of thecrosspoints in the oflice matrix and additional mark orders aredelivered to the skew link mark selectors to close appropriatecrosspoints in the interprimary matrix to effect a connection betweenthe calling party and the called party over an idle interprimary skewlink path, lfor example link 78. Conversation may then ensue for anindeterminate period and release is ellected in the manner describedabove for FIG. 2.

lt will be appreciated that in addition to the obvious advantagesattendant upon connection of PB-X subscribers connected to the same orother primary concentrators, similar benefits may -accrue to otherparties having a high community of interest such as calls betweentenants of an apartment dwelling and the superintendent or manager ofthe building Whose telephones are connected to the same primaryconcentrator or diierent primary concentrators interconnected throughskew links but who are not necessarily connected to a PBX.

lt is to be understood that the above-described arrangements areillustrative of the application of the principles of this invention.Numerous other arrangements may be -devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

l. A telephone line concentrator system including a plurality of lines,a central office, a plurality of office -links less in number than saidlines extending `from said central oice, a line concentrator remote fromsaid office =for extending calling ones of said lines to said ofce oversaid ofiice links .under control of said central oiice, means at saidofiice for interconnecting a calling one of said lines and a called oneof said lines over a path through said otlice including said oice links,and a plurality of intraconcentrator links within said concentrator torthereafter interconnecting said lines under control of said centralofiice.

2. An automatic telephone line concentrator system includinga pluralityof lines, a central oice, a plurality of oftice links less in numberthan said lines connected to said central office, a plurality of lineconcentrators -for connecting calling ones of said lines to said officelinks under control of said central oice, and a plurality ofinterconcentrator links for interconnecting said lines under control toflsaid central office.

3. A telephone line concentrator system including a plurality of lines,a central oice, a plurality of oiiice links less in number than saidlines connected to said central oilice, a plurality of concentrators forextending calling ones of said lines to said oice links under control ofsaid central office, a plurality of intra-concentrator links within saidconcentrators ttor interconnecting said lines under control of saidcentral oiiice, and a plurality of interconcentrator links forinterconnecting said concentrators and said lines under control of saidcentral office.

4. An automatic telephone line concentrator system including a pluralityof lines, a central oiiice, a plurality of oice links less in numberthan said li-nes extending from said central oicc, a plurality of lineconcentrators for connecting said lines to said links under control ofsaid central office, a plurality of interconcentnator links forinterconnecting said lines under control of said central oce, meansresponsive to a calling condition on one of said lines yfor extending aconnection from said line over a rst concentrator and a first oce linkto said central oice, additional means responsive to signals from saidcalling line representative of a called line :for 'extending aconnection from said central roice to said called line over a second ofsaid oflce links and a second concentr-ator, and means responsive toanidle condition on said called line :for disconnecting said iirst andsecond oflice links and for transferring sa-id connection -to an idleone of saidinterconcentrator links to provide a talking path 1.3 betweensaid lines exclusive of said l-inks to said central toice.

5. An automatic telephone line concentrator system including a pluralityof li-ne-s, a central office, a plurality of office links connected tosai-d central oiice, said links being less in number than said lines, aline concentrator for connecting said lines to said links under controlof said central oice, a plurality of intra-concentrator links Withinsaid concentrator fior interconnecting said lines under control of saidcentral otiice, means in said central oiiice responsive to signals onone of said lines representing another of said lines for establishing aconnection between said lines over said oice links to said centralotlice, and ymean-s for thereafter releasing said office links to saidcentral ofiice and transferring the connection to an idle one of saidintra-concentrator links to provide a talking path between said linesexclusive of said oice links to said central oiiice.

6. A telephone line concentrator system including a plurality of lines,la central office, a plurality of office links less in number .than saidline-s, primary concentrator means for connecting said lines to saidlinks under control of said central roiiice, .a plurality of trunks lessin number than said links and connected to said central oice, asecondary concentrator for connecting said links to said trunks undercontrol of said central office, and a plurality of intra-primary locallinks Within said primary concentrator -for interconnecting said linesunder control of said central -oice to provide a talking path betweensaid lines exclusive of said oiice links and said trunks to said centralotiice.

7. An automatic telephone switching system including a .plurality oflines, a central oce, a plurality of office links less in number thansaid lines, a plurality of primary concentrators for connecting saidlines to said office links, a plurality of trunks less in number thansaid office links connected to said central oice, a plurality ofsecondary concentrator means for connecting said links to said trunksunder control of said central oice, said primary concentrators includinga plurality of interprimary local links connected between said primaryconcentrators for interconnecting said lines under control of saidcentral office.

8. An automatic telephone line concentrator system including a pluralityof lines, a central oiiice, a plurality of oiiice links less in numberthan said lines, a plurality of primary concentrators for connectingsaid lines to said oiiice links under control of said central office, aplurality of trunks less in number than said oflice links and 4connectedto said central oiiice, a plurality of secondary concentrator means forconnecting said oiiice links to said trunks under control of saidcentral oice, a plurality of interprimary local links coupling saidprimary concentrators for interconnecting said lines under control ofsaid central otiice, and a plurality of intra-primary local links withinsaid primary concentrators for interconnecting said lines under controlof said central office.

9. A two-stage telephone line concentrator system including a pluralityof lines, a central otce, a plurality of oice links less in number thansaid lines, a plurality of primary concentra-tors `for connecting saidlines to said oflice links under control of said central office, aplurality o-f trunks less in number than said oice links and connectedto said central Ioiiice, secondary concentrator means for connectingsaid office links to said trunks under control of said central oice,said primary concentrators including a plurality of intra-concentratorlocal link connections for interconnecting said lines under control ofsaid central oice, means responsive to a calling condition on one ofsaid lines lfor establishing a connection from said line to said centraloffice over a path including a first of said primary concentrators and afirst of said oiiice links, additional means responsive to signals fromsaid calling line representative of a called line connected to saidtirst concentrator -for extending the connection from said centraloliice to said called Iline over a path including a second of saidoiiice links and said first pri-mary concentrator, and means yforthereafter releasing said iirst and second office links and fortransferring the connection to an idle one of said intra-concentratorlinks to provide a talking path between said calling and called linesexclusive of any channels to said central oilice.

10. An automatic telephone line concentrator system including aplurality of lines, a central oliice, a plurality of office links lessin number than said lines, a plurality of primary concentrators -forconnecting said lines to said links under control of said centraloiiice, a plurality of trunks less in number than said links connectedto said central oice, secondary concentrator means for connecting saidoffice links to said trunks under control of said central office,intra-primary concentrator local links Within said primary concentratorsfor interconnecting said lines under control lof said central office, aplurality of interprimary local links for coupling said primaryconcentrators and tor interconnecting said lines under control of saidcentral office, means responsive to a calling condition on one or saidlines ttor extending a connection trom said line to said central loice,means responsive to signals from said calling line representing a calledline for extending a connection over one of said office links and one ofsaid trunks to said central -oice and further over another ot saidtrunks and another of said cnice links to sai-d called line, and meansfor thereafter releasing said office links and trunks and fortransferring the connection to one of said interprimary local links toprovide a talking path between said calling and called lines exclusiveof channels to said central office.

ll. A telephone line concentrator system including a plurality -oflines, a central oiiice, `a lesser plurality of office links extending`from said central oiice, a plurality of lin-e concentrators diorconnecting said lines to said ottce links under control of said centraloiiice, a plurality of intra-concentrator local links within saidconcentrators for interconnecting said lines under control of saidcentral oliice to provide a communication path exclusive of said oicelinks, scanning means at said concentrator-s for examining the servicecondition of vsaid lines and olice links and tor supplying indicationsthereof to said central otlice, and additional scanning means at saidconcentrators for examining the service condition of saidintraconcentrator local links and tor supplying indications thereof to`said central oliice.

12. A telephone line concentrator system in accordance with claim llincluding in addition a plurality o-f interconcentrator local linkscoupling said concentrators for interconnecting said lines under controlof said central office to provide a communication path exclusive ot saidoffice links, said additional scanning means being connectable to saidinterconcen-trator local links to examine the service condition thereofand to supply indications 4thereof to said central oliice.

13. A two-stage telephone line concentrator system including a pluralityof lines, a central oiiice, a plurality of oice links less in numberthan said lines, primary concentrator means Ifor connecting said linesto said oice links under control of said central oiice, a plurality oftrunks less in number than said :office links and connected to saidcentral oiice, secondary concentrator means for connecting said oiicelinks to said trunks under control of said central ofce, said primaryconcentrator including a plurality of intra-concentrator local links forinterconnecting said lines under control ci said central otlice toprovide a communication path exclusive of said o'ice links and trunks,scanning means at said primary concentrator for examining the servicecondition of said lines and links and for supplying indications thereofto said central oiiice, and additional scanning means at said primaryconcentrator tor examining the service condition of saidintra-concentrator local links and for supplying indications thereof tosaid central oice.

14. A two-stage telephone line'concentrator system in accordance withclaim 13 including in addition a plurality of interconcentrator locallinks coupling said concentrators for interconnecting said lines undercontrol of said central oice to provide communication paths exclusive ofsaid oice links and trunks, said additional ysca-nning means beingconnectable to said interconcentrator local links to examine the servicecondition thereof and to supply indications thereofi to said centraloice.

15. A telephone line concentrator system including a 4plurality oflines, a central oice, a plurality of oilice links less in number thansaid lines and connected to said central office, a plurality of lineconcentrators for connecting said lines to said oiiice links undercontrol of sa-id central oice, and a plurality of interconcentratorlocal links coupling said concentrators to interconnect said lines undercontrol of said central oiice to provide talking paths exclusive of saidoice links, scanning means at said concentrators for examining theservice condition of said lines and ofce links and for supplyingindications thereof to said central oilice, additional scanning means atsaid concentrators for examining the service condition of said locallinks and for supplying indications thereof to said central ofce, linemark selector means and link mark selector means at said concentratorsindividually connectable to said lines and office links to effectconnections therebetween, link release means at said concentratorsindividually connectable to said oice links to release said connections,local link mark selector means at said concentrators individuallyconnectable to said local links to effect connections between saidrlines and local links, and local link release means at saidconcentrators individually connectable to said local links to releasesaid intercon- -centrator connections.

16 cations thereof to said central oice, and said local link markselector means and local link release means being additionallyconnectable to said intra-concentrator local links to eiiectuateconnections between said local links and said lines and to release saidconnections.

17. A two-stage telephone line concentrator system including a pluralityof lines, a central otiice, a plurality of oice links less in numberthan said lines, a plurality of primary concentrators for connectingsaid lines to said oice links under control of said central ofce, aplurality of trunks less in number than said office links and connectedto said central oflice, secondary concentrator means for connecting saidofce links to said trunks under control of said central oiice, saidprimary concentrators including a plurality of interconcentrator locallink connections for interconnecting said lines under control of saidcentral oliice to provide conversation paths exclusive of said oincelinks, scanning means at said primary concentrators for examining theservice condition of said -lines and links and for supplying indicationsthereof to said central oiiice, additional scanning means at said pri--mary concentrators for examining the service condition of saidinterconcentrator local links and for supplying indications thereof tosaid central oflice, line mark selector means and office link markselector means at said primary concentrators individually connectable tosaid lines and oiiice links to eifect connections therebetween, officelink release means at said primary concentrators individuallyconnectable to said office links to release said connections, local linkmark selector means at said primary concentrators individuallyconnectable to said local links to effect connections between said linesand local links, and local link release means at said primaryconcentrators individually connectable to said local links to releasesaid connections.

References Cited in the iile of this patent UNITED STATES PATENTSlWochinger Jan. 3, 1939

1. A TELEPHONE LINE CONCENTRATOR SYSTEM INCLUDING A PLURALITY OF LINES,A CENTRAL OFFICE, A PLURALITY OF OFFICE LINKS LESS IN NUMBER THAN SAIDLINES EXTENDING FROM SAID CENTRAL OFFICE, A LINE CONCENTRATOR REMOTEFROM SAID OFFICE FOR EXTENDING CALLING ONES OF SAID LINES TO SAID OFFICEOVER SAID OFFICE LINKS UNDER CONTROL OF SAID CENTRAL OFFICE, MEANS ATSAID OFFICE FOR INTERCONNECTING A CALLING ONE OF SAID LINES